Commercial drilling applications, such as rock drilling for oil and gas typically use elongated drillstrings having a drill bit attached thereto. During drilling the drillstrings often experience vibrations, such as self-excited vibrations (chatter) in the longitudinal direction (axial chatter), rotational vibration and bit whirl. The vibrations in the bit of a drillstring may produce an undulated surface in the rock and/or cause failure of the cutter portion. In particular, failures of Polycrystalline Diamond Compact (“PDC”) cutters and damage to the bottom hole assembly are of concern.
Even though there are many types of vibrations encountered during drilling, chatter is considered to be one of the main causes of failure in PDC bits, particularly in hard rock formations such as Sierra White Granite (“SWG”). When the PDC bits fail a new bit must be installed, requiring removal of the drillstring from the drilling hole. Removing the drillstring halts drilling, which increases operational cost and repair time.
It has been shown that the proper combination of weight on bit (“WOB”), rotating speed, and bit design reduces the axial chatter. However, the bit design is not adjustable or variable under field conditions and poor surface telemetry with bottom hole conditions often results in the actual downhole dynamic conditions being incorrectly diagnosed. This is particularly important as the dynamics of drillstrings are constantly varying with the drilling depth. Moreover, rock properties vary and unexpected variations in rock hardness is common. Sudden changes in load as the bit transitions a soft-to-hard rock boundary is often sufficient to induce drillstring instability.
A suppression assembly and drilling tool for suppressing drilling vibrations that do not suffer from one or more of the above drawbacks would be desirable in the art.